Method for heat treating semiconductor material using high intensity CW lamps

ABSTRACT

Apparatus for annealing semiconductor wafers includes a support for receiving the wafers and resistive heaters for heating the wafers by thermal conduction through the support or by convection. A high intensity arc lamp scans the heated wafers thereby raising the temperature sufficiently for heat treating. The process is simple, rapid, efficient, and does not create damaging thermal stresses in the wafers. The high temperature and short time treatment enables material properties unobtainable with conventional thermal processes.

This invention relates generally to semiconductor technology, and moreparticularly the invention relates to heat treating of semiconductorwafers.

Electronic devices are formed in a single crystal semiconductor wafersby the selective introduction of dopant atoms into the lattice structureof the semiconductor material. Group III elements of the periodic table,such as boron and gallium, when diffused or implanted into thesemiconductor lattice structure render the semiconductor material P typesince these elements are accepters of electrons in the atomic valencebands of the elements. Group V elements of the period table, such asphosphorous and arsenic, when introduced into the semiconductor latticestructure render the semiconductor material N type since the elementsare donors of electrons from the valence bands of the atoms.

The dopant atoms can be introduced into the semiconductor material bydiffusion from a dopant atmosphere in a diffusion furnace or by ionimplantation in which charged dopant ions are driven into thesemiconductor material by a particle accelerator. Particularly in ionimplanted semiconductor material lattice defects result and requirethermal annealing to properly orient the dopant atoms in the latticestructure.

Additionally, polycrystalline silicon is heat treated by a furnace or bylaser scanning and the like to increase crystal grain size and also toactivate dopants in the polysilicon.

Heretofore, thermal annealing of semiconductor material has beeneffected in a furnace with temperature cycled to 700° C.-1100° C. toeffect activation of the implanted ions in the semiconductor lattice orto increase grain size in polycrystalline material. This procedure istime consuming and results in a diffusion or migration of the dopantatoms with decreasing performance of the semiconductor product.

More recently, laser annealing has been introduced. Laser annealingallows nearly instantaneous heating and cooling of the semiconductormaterial with reduced dopant ion migration within the semiconductorlattice structure. However, because of the small beam of the laser,considerable time is necessary for the total scanning of thesemiconductor wafer. Moreover, laser equipment is expensive and veryinefficient in power usage. Further, laser annealing equipment as wellas annealing furnaces require considerable space in the clean roomatmosphere of a semiconductor production area.

Thermally assisted flash annealing using high intensity xenon flashlamps has been proposed. However, the short pulses of incoherent lightinduce a very short temperature rise in the material. To observe anyannealing effect, the sample must be heated considerably (approximately600° C.). Also since the energy discharged into the lamp is limited,only very small areas (e.g. 1 cm×1 cm) can be annealed. The resultantmaterial was reported to contain defects in a concentration thatindicates incomplete annealing. Hot filament ribbons have been proposed.This scheme is intended to be used for the production of silicon whichis deposited on a substrate for solar cell usage. In this application itis intended to melt the amorphous silicon with a hot tungsten filamentand let this molten silicon cool down and recrystallize. All theapparatus has to be in vacuum to inhibit the oxidation of the hotfilament. Because of the vacuum no preheating of the material isproposed and therefore the large gradient between the molten surface ofthe substrate may include strain and stress in the material.

Accordingly, an object of the present invention is an improved method ofheat treating semiconductor material.

Another object of the invention is apparatus for quickly annealing dopedand undoped semiconductor material.

Still another object of the invention is apparatus which is relativelysimple and inexpensive and which requires little space in asemiconductor manufacturing facility.

Briefly, in accordance with the invention a support is provided forholding a semiconductor wafer for heat treatment. The support includesheater means for heating the wafer to an elevated temperature below thetemperature for heat treatment and below a temperature which causesmigration of dopant atoms in the semiconductor lattice structure. A highintensity incoherent CW light source is positionable with respect to thesupport for irradiation of a semiconductor body held on the support.Means can be provided for varying the spacing between the light sourceand the support, and means is provided for effecting relative motionbetween the light source and the support whereby the surface of thewafer can be scanned by the light source. Means is provided to controlthe light intensity by varying the current through the lamp.

Preferably, the high intensity light source comprises a CW arc dischargetube of sufficient length to scan the entire width of a semiconductorbody. The support preferably includes a vacuum chuck for holding asemiconductor wafer, and resistive heater means are embedded in thesupport for heating of the wafer by thermal conduction through thesupport.

In another embodiment of the invention, the substrate is placed onisolated pins but close to the heater surface. The semiconductormaterial is heated to the heater temperature. When the lamp is scanning,the wafer is far enough from the heater so that its temperature can risequickly and independently of the chuck temperature throughout because ofheat conduction. This reduces thermal gradients and facilitates theincrease in temperature for a given light intensity.

In annealing a semiconductor wafer and the like, the high intensitylight source is scanned across the surface of the wafer at a speeddetermined by the spacing between the light source and wafer and by theparticular temperature desired for annealing the semiconductor materialand the preheated temperature of the wafer. Thus, a semiconductor wafercan be annealed in a matter of seconds thereby increasing the throughputof annealed semiconductor material and without introducing residualstresses in the annealed wafer and without causing dopant migration.Moreover, polycrystalline silicon is readily recrystallized withincreased crystal size.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and appended claimswhen taken with the drawing, in which:

FIG. 1 is a perspective view of one embodiment of apparatus inaccordance with the invention.

FIG. 2 is a perspective view of another embodiment of apparatus inaccordance with the invention.

FIG. 3 is a plot of dopant concentration of three wafers as implantedafter conventional thermal annealing and after processing in accordancewith the present invention.

In the drawing, a semiconductor wafer to be annealed is placed on asupport pedestal 8 within housing 10 and positioned by means of a chuck12 through a vacuum line 14. The support 8 is a heat conductive materialsuch as brass with a graphite top plate in which a plurality ofresistive heaters 16 are provided with the resistive heatersinterconnected with an electrical power source through lines 18. In oneembodiment the resistive heaters are CalRods and a sufficient number ofheaters are provided whereby the support 8 and a wafer maintained onvacuum chuck 12 can be heated to 500° C. The support 8 is verticallymoveable by means of lead screws 20 which are driven by suitable motordriven gear train shown generally at 22. Alternatively, support 8 can bemade stationary.

Mounted within housing 10 above pedestal 8 is thermal radiationapparatus 30 including a radiant heater 32. A concave surface 34reflects radiant energy downwardly onto the semiconductor body. In apreferred embodiment the radiant heater comprises a CW arc dischargetube such as the dc crypton arc discharge tube FK-111C-3 available fromEG & G. The tube 32 is of sufficient length to irradiate the entiresurface of a semiconductor wafer on a vacuum chuck 12 in a single scan.

The radiant heater apparatus 30 is moveably mounted on a pair ofhorizontal rails 36 and is driven by means of motor 38 and lead screw40. Thus, the radiant heater 30 can be moved from one side of thehousing assembly 10 to the other side to thereby scan a wafer onpedestal 8. Alternatively, the radiant heater 30 can be fixed and thesupport 8 moved along rails.

In annealing a semiconductor wafer in accordance with the presentinvention, the current through the radiant heater 32 is selected alongwith the linear speed of heater assembly 30 whereby the surface of thesemiconductor wafer is heated to sufficient temperature for annealing.Normally, the annealing temperature for silicon semiconductor materialwill be in the range of 1200°-1400° C. In some applications highertemperatures and melting can be achieved. Importantly, by preheating thesemiconductor wafer 12 to a temperature of about 500° C. by means of theresistive heaters 16, excessive thermal stresses in the semiconductorwafer are avoided since the temperature rise provided by the radiantheater 30 need be only 700°-900° C.

In the embodiment of FIG. 2, a semiconductor wafer 50 is supported on aplurality of thermally insulating ceramic posts 52 above the vacuumchuck 54. By reversing the air flow in the chuck, uniform heating of thewafer is facilitated by convection heating. In some cases convectionheating is sufficient and reverse air flow is not necessary. Thus, asthe lamp is scanned across the wafer surface, temperature can increasethroughout the thickness of the wafer and temperature gradients in thewafer are minimized.

FIG. 3 is a plot of dopant concentrations in three identical wafers twoof which were annealed by conventional thermal processing and by thescanning process in accordance with the present invention. The dopantprofile 60 for the conventionally processed wafer shows considerabledopant migration during annealing as compared to the dopant profile 62for the scanned wafer. This profile 62 is identical to the as implantedprofile 61 meaning that annealing with the present invention does notalter the dopant concentration profile. Prior to annealing all wafershad sheet resistivity of 3100 ohms per square. After conventionalannealing, one wafer had sheet resistivity of 150 ohms per square, whilethe scanned wafer had sheet resistivity of 168 ohms per square. Thedifference stems from the fact that the thermal annealing profile ismore diffused and has slightly higher average mobility. In both casesall the dopants are active and contribute to the electrical conductivityof the crystal.

The annealing or heat treating of semiconductor wafers using theapparatus and method in accordance with the present invention has provedto be advantageous in initial cost of the equipment, limited spacerequired in the semiconductor manufacturing facility, and semiconductormaterial throughput. Also, steeper and shallower junctions are achievedmeaning potentially smaller and faster devices. Also since achievablescanning temperature ranges are higher than furnace temperatures largerpoly grain can be grown with the above method. By preheating thesemiconductor wafer through the support pedestal prior to the radiationbeam scanning, deleterious thermal stresses within the semiconductorwafer are avoided and the light intensity needed to raise thesemiconductor temperature is reduced considerably. The resultingannealed wafers are high quality. The invention has heat treatingapplications other than annealing and including polycrystallinesemiconductor regrowth to increase crystal grain size, aluminumsintering and grain growth, phosphorous glass reflow and the like.

Thus, while the invention has been described with reference to aspecific embodiment, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method of heat treating a semiconductor body ata high temperature for a short duration comprising the stepsofpreheating said semiconductor body to a first temperature, andradiating a surface of said semiconductor body with a high intensity CWlamp thereby rapidly heating said radiated surface to a secondtemperature higher than said first temperature for a short duration oftime.
 2. The method as defined by claim 1 and further including the stepof adjusting the spacing between said semiconductor body and said lampand the scan speed whereby said semiconductor body is heated to apreselected temperature by said lamp.
 3. The method as defined by claim1 and further including the step of adjusting the power to said lampwhereby said semiconductor body is heated to a preselected temperatureby said scanned light source.
 4. The method as defined by claim 1wherein the step of preheating said semiconductor body includes mountingsaid semiconductor body on a support and heating said support.
 5. Themethod as defined by claim 1 wherein the step of preheating saidsemiconductor body includes positioning said body in spaced relationshipwith respect to a top surface of a support pedestal whereby said body isheated by convection.
 6. The method as defined by claim 1 wherein saidstep of radiating a surface includes scanning said surface with saidlamp.